With rapid progress in digital world, recording medium of large storage capacity, small in size, and low production cost is demanded. Among them, optical recording medium is the most popular one due to its large storage capacity. Such optical recording medium is commercial available as an optical disc which is recorded by laser beam. Such an optical recording medium is mainly classified into two kinds, one is a recordable optical disc on which information can be recorded and played and the other one is a rewritable optical disc on which information can be recorded, erased, and played. The recordable optical disc is much popular due to its low cost and excellent stability.
Regarding the recordable optical disc, it is further classified into DVD-R and CD-R, and the storage capacity for DVD-R is about 7 times of that for CD-R. The wavelength of the laser for recording and playing DVD-R is about 650 nm so that an organic dye having strong absorbance in the range of 550-620 nm can be used as a recording layer for DVD-R disc. For using in optical disc, the dye is required to have a good solubility in organic solvent, especial in alcohol solvents, an excellent film-forming ability, a good stability, and a high reflectivity when forming into a film. Currently, the DVD-R disc has advanced to high writing speed, for example, increased from 1×˜8× to 12×˜16×.
Properties of the recording material in the recording layer are more important when the recording disc is recorded at a high writing speed. The laser power for recording would be increased when the recording is carried out at a high writing speed in order to record information on the disc in short time. However, the larger laser pulse will result in expending of pit laterally when it illuminates on the disc, even damage the trench on the disc or destroy the information recorded on the disc. However, in case of that the writing is carried out at a high writing speed, if the recording material in the recording layer can be written at a low laser power, the limitation on recording machines and recording mechanism will be eliminated. A low jitter value of the recording material is also required. Therefore, to work well at high writing speed is to develop a novel recording material which possesses the following properties required in high writing speed: low burning power, low signal jitter, good film-forming ability, be soluble in the used solvent, and low exothermic heat.
There are several dyes used in recording material for optical recording medium currently, one of them is a cyanine compound. The cyanine compound is the first one successfully used as recording material in CD-R optical disc, and thus various cyanine derivatives have been widely developed and used as recording material in DVD-R optical disc, for example, those disclosed in Taiwan Patent Publication No. 591646, Taiwan Unexamined Patent Publication Nos. 200523917, 200306334, 200508325, 200519166, 200608384, 200613409, 200621710, and WO 2006/123807, and EP 154723A2. Physical properties of the cyanine derivatives can be modified by substituting functional groups on the molecular when using in recording material on optical recording medium, such as changing the light-absorbing wavelength, changing decomposing temperature, and endothermic and/or exothermic decomposition. However, such physical properties are not necessary relevant to the molecular structure and are screened certainly by experiments to select the suitable ones.